The present invention relates to a jam prevention mechanism for a heat roll fixing unit used in an image formation device, such as a printer, employing so called electrophotographic system, and more particularly to a jam prevention mechanism for preventing a recording sheet, on which a visible image has been formed, from being caught around a heat roller in a sheet discharge process after the recording sheet has been fixed, by which jam is caused.
Conventionally, there has been known a copy machine and a printer as an image formation device using so-called electrophotographic system, wherein a latent image is formed by exposing a photoreceptor on the surface of a photoconductive drum, adhering toner to the latent image to change the latent image to a visible image, and then transferring the toner image to a recording sheet and fixing the transferred image by a fixing unit.
In this type of image formation device using the electrophotographic system, a so-called heat roll fixing method is used, by which an unfixed visible image on the recording sheet is pressed against a heat roller, i.e., a heat roller, and toner is heated and fusedly adhered by the heat applied from the roller.
To effect the heat roll fixing, a pair of fixing rollers are composed of a heat roller heated to high temperatures and a press roller, which are disposed in parallel to each other with the recording sheet held therebetween to cause the recording sheet to be pressed against the heat roller for heating; the heat roll fixing is advantageous in that the fixing can be carried out at a high speed with a high thermal efficiency. Usually, the heat roller is driven by a driving source and the press roller is arranged to be rotated with the heat roller, and the recording sheet is fed through a gap generated between the pair of fixing rollers.
Incidentally, a guide member is adjacently disposed at the sheet discharge side of the above-mentioned heat roll fixing unit with the pair of fixing rollers so that the recording sheet is separated from the heat roller after the fixing operation and guided along the desired feeding path. Accordingly, the recording sheet is prevented from being caught around the heat roller, and being jammed.
A gap between the guide member and the rollers is preferably minimized in view of the function thereof. In other words, it is preferable to separate the recording sheet from the heat roller immediately after the fixing operation is finished.
Thus, a claw-shaped separation member is often provided at the extreme end of the roller side of the guide member and the separation member is urged against the roller by a predetermined pressing force. Further, the separation member may be composed of an elastic material so that the separation member is pressed against the roller by the returning force thereof after the separation member has been deformed. Note that the larger the pressing force applied to the roller by the separation member, the more effectively the recording sheet is prevented from being caught around the roller.
However, in the arrangement in which the press roller is arranged to be rotated with the driven heat roller as described above, a problem arises in that the pressed separation member resists the rotation of the press roller arranged to be rotated with the heat roller, with the result that the recording sheet is poorly fed and the visible image is poorly fixed.
To cope with this problem, when the pressing force of the separation member toward the press roller is reduced, the effect to separate the recording sheet, i.e., the effect to prevent the recording sheet from being caught around the roller is deteriorated, and thus it is very difficult to set a pressing force of the separation member to such a state, wherein the pressing force does not resist the rotation of the press roller and sufficiently prevents the recording sheet from being caught.